Blog for the RepRap project at www.reprap.org - a project to create an open-source self-copying 3D printer. To get all the early posts on this blog with all the images as a single PDF visit this page.

Monday, September 04, 2006

Use of Castable Bearings in RepRap

Apologies for the delay in posting this....

A few weeks ago I attempted to produce some bearings using Field's metal and RP moulds. The aim was to prove that it was possible for a RepRap machine to produce its own bearings, as Field's metal melts at approximately 70 degrees celcius......

I tried 2 methods1. Casting the metal in a mould with the 8mm steel bar in situ, using silicone grease as a spacer.

2. Casting the metal in a mould with a plug that could be hammered through once the metal had solidified.

Method 1 failed as a bearing (but would be good as a locating/fixing mechanism - ie calibratemachine, then set all joints in metal)

Method 2 produced some nice bearings..... (Only tested by hand so far)

The Moulds

Method 1 - 3 hole mould and cap used to locate the rod and seal the base.

Method 2 - 6 hole mould with varying plug diameters, the support material needs to be left on to hold the plug in place.

Wow! Once Vik sees this he'll probably start wondering if he can get Zaphod to build cylinders.

If one was to build a two-part mold in the shape of a "choked" cylinder out COPA, and then press/surround the resultant part with a straight walled COPA tube, you could use a the casting technique to form a self-aligning bearing.

Fields Metal, and other low-temp eutectic metals like it, are heavily used in precision machining and in optics to protect or fixture parts in place.

Castable bearings are pretty useful thing to add to the RepRap toolbox. Just to save some reinventing the wheel, it might be a good idea to investigate a technological precursor to ball-bearings called 'Babbitt' bearings (Google is your friend..) for the bearing shell designs. The metal used is probably moderately unsuitable for use with COPA, but the concepts translate.

Sorry I haven't been around much for a while, but things (class, work, more work, and even more work.. 100+ hrs of overtime in the last two weeks alone) have kind of gotten in the way of fun things.

That's interesting! So how does the Fields metal stand up as a bearing, does it wear well? In my limited experience, the hardest part is not basic fabrication of bearings and such, but getting the tolerances and surfaces good enough to get long lasting parts. But with a few well chosen tricks...

I suppose methode 1 failed because of thermal contraction. I've run into that with metal plugs surrouned by cast metal. In some cases it can be drilled out to size afterwards if the plug is pushed out.

The thing about a babbitt-type bearing, especially one done with Feilds Metal, no-lead low-temp solder, or any similar material is that you can effectively fabricate a custom bearing to the shape/size and characteristics you want. You don't need machine tools to make the bearing or the shaft. Babbit can be poured with a ladle and melted on a charcoal grill. Carving it to make smooth surface can be done with a good work knife.

Vic, that's exactly how Babbitt bearings usually are made, there is a good description on Wikipedia.

I think straight Babbitt bearing emulation is rather labor intensive for the target RepRap concept though. Instead, I'd suggest experimenting pouring against a very smooth shaft, but with a cut at the top to allow contraction. I'd also suggest a very thin layer of Field's metal, supported by a strudy extruded plastic "pillow".

Maybe it could even be poured thin and flat, say on a piece of glass, then rolled to the desired diameter.

In any case, if the shaft is not perfectly smooth, nothing bearing-like will last very long. But that might be okay for some uses.

Actually...what about using a technique like this to make a threaded rod not threaded? Or to make a low-backlash nut... wrap a few turns of heavy-gauge solder around the rod, put a holdar around those... it's seeming to work when I try it. Also, wrapping molten solder around one of those multi-thread screw drives that come on the steppers I have results in a passable nut for them.